The problem is to monitor the presence of a fluid at one or several different places and to report this information to a distant central site. Many documents of the prior art relate to the detection of the presence of fluid using sensors. Other documents describe the use of passive or active RFID tags on objects to collect information by means of RFID readers. The problem is to find a system:                capable of detecting a fluid on an object or person in one or different places;        capable of sending this information and other data to a distant site;        permanently easily identifiable;        autonomous in terms of energy;        easy to build;        cheap enough to be disposable.Detection of a Liquid Using Non Contact Tags        
US patent application 2004/0070510 entitled “Radio frequency wetness detection system” discloses a detection system used in, e.g. diaper for detecting the presence of urine, includes passive resonant tag having substrate, inductor, and capacitor, and a remote detection mechanism having transmitter, alarm, and antenna. A moisture monitoring device is provided having a resonant tag and a remote detector. The resonant tag comprises a plastic substrate having holes punched through it, a capacitor plate on one side that is connected to a second capacitor plate and inductor coils on the other side. The resonant tag comprises a live LC circuit having a natural resonant frequency that does not activate the detector. The resonant tag is affixed to absorbing materials in a diaper or other sanitary undergarment. Upon the introduction of moisture into the circuit, the resonant frequency is eliminated as the circuit is deactivated and an alarm in the detector is triggered. At this point it would be apparent to a parent or caretaker that the diaper is wet and in need of replacement.
In US patent application 2004/0070510 the effect of the liquid detection is to de-tune a resonant circuit. The information provided by this system is a binary information “on” or “off”. No data is transmitted by the detector. This prevents the system to automatically discriminate, from a remote location, several systems detecting moisture at the same time.
Japanese patent JP2005000602 entitled “Wet Diaper Informing and Indicating Device and Diaper Equipped with Wet Detecting Device” discloses a system to provide a wet diaper informing device and a diaper equipped with a wet detecting device which allow a care-giver to determine easily whether a user of the diaper is within a tolerable limit when the exchange of the diaper is delayed by informing the wet status of the diaper, such as the number of times the diaper gets wet or the time elapsed since the diaper got wet. The wet diaper informing device and the diaper equipped with wet detecting device comprise a wet detecting device and a display device. The wet detecting device has a radio frequency-identifier (RF-ID) tag comprising an IC chip having a data storing medium for storing the number of times that wet signals are generated or the time elapsed since a wet detecting signal was entered, a printed antenna and a communication control section. A control section transmits data stored in the data storing medium to the communication control section via the antenna. The display device comprises a communication control section, a control section, an antenna and a display section. The display section displays, in response to the transmitted data, the number of times that the wet signals are generated or the time elapsed since the wet detecting signal was entered.
In Japanese patent JP2005000602, the system is not autonomously powered. The wet detector is just providing information to the system, but does not provide any power to the tag that only operates as a passive tag (short distance reading).
Powering of Wetness or Liquid Sensors
U.S. Pat. No. 5,959,535 entitled “Electrogalvanic-powered diaper wetness sensor” Remsburg Ralph, discloses an electrical wetness sensor powered by the electrogalvanic action of an anode, a cathode and an electrolyte. In the preferred embodiment the electrolyte is urine and the device is used to warn of incontinence. This device contains the following features: sensitivity to wetness; electrical battery-less operation; hermetic and permanent sealing; no-maintenance operation; portability, inexpensiveness, and disposableness. The alarm unit may be configured to produce a visual and/or audible signal. In the preferred embodiment the alarm unit transmits a signal to a remote FM radio receiver or dedicated device to announce incontinence. Incontinence is announced by a tone, visual signal, silencing of the radio station the receiver is tuned to, or activation of a microphone within the alarm unit to allow remote listening.
U.S. Pat. No. 5,959,535, does not use an RFID system and therefore, cannot provide means to read the stored data as long as the battery is not in operation. Moreover, for distant operation, U.S. Pat. No. 5,959,535 uses an FM radio signal, which transmits just an alarm. No additional information, like localisation is transmitted. Moreover, adding an FM modulator is expensive.
Japanese patent JP2000329732, Okuyama Yuji, entitled “Excretion detecting device” discloses a detection device that is safe for a user and enables a detection part to be used easily again by using a battery that is solid in a dry state, uses an electrolyte for indicating acidity when being dissolved in water, and generates power when water is added. The detection device for reporting excretion when a person wearing a diaper performs excretion is provided with, for example, a cathode, a dried electrolyte, an anode, and a transmitter. The transmitter and an electric wire are protected from direct urine while being covered, for example, with a resin case. Zinc or magnesium, citric acid, and carbon, copper, silver, or gold can be used as the cathode, an electrolyte, and the anode, respectively. A weak acid other than the citric acid that is solid when it is dry and is not harmful to a human body may be used. Then, when a battery is fitted to a specific position and is exposed to urine, water enters the electrolyte and power is generated and then an electronic wave is emitted from the transmitter. By opening the resin case or the like, the part of the electrolyte and the cathode can be exchanged.
Japanese patent JP61167854 entitled “Liquid sensor” discloses a system to eliminate the need for having a power source device on the outside or in the outside and to obtain a liquid sensor which is small in size, inexpensive and is suitable for a throw-away application by constituting a primary battery of plural electrodes consisting of different materials and electrolyte in a detecting part. The comb-shaped electrodes which constitute a positive electrode and negative electrode are formed of the different materials on a substrate. The electrodes are formed of, for example, carbon for the positive electrode and Ni for the negative electrode. A water absorbent consisting of, for example, PVA powder and starch is formed like a film onto the substrate contg. the electrodes. The electrolyte such as NH4Cl and depolarizer such as MnO2 are mixed with the absorbent. The primary battery is thereby constituted of the electrodes and the detecting part consisting of the absorbent which is a mixture composed of the electrolyte and depolarizer. The primary battery functions when water or urine is adsorbed to the absorbent. Then the electrolytic NH4Cl induces an ionization reaction and the current flows between the electrodes. The detection circuit is driven by the electromotive force thereof.
Japanese patents JP2000329732, JP61167854 describe only a detection device and liquid sensor powered by a battery activated by a liquid. This detection device or liquid sensor is not based on the RFID technology and cannot transmit in a wireless way to a remote system data (for instance the position) related to tagged objects.
Japanese patent JP4138155 entitled “Urinary Incontinence Detector” discloses a system to suppress the consumption of a power source voltage and enhance the life by assembling an urinary incontinence detecting member by two kinds of different metallic materials producing an electromotive voltage by a potential difference caused by urine to form a pair of metallic terminals, capable of mutually approaching and separating for operating an urinary incontinence detecting circuit on the basis of the electromotive voltage. Metallic terminals are inserted to a determined position of a paper diaper which is worn by a patient, and the remote distance between the metallic terminals is set to a desired value according to an urinary incontinence state such as urine quantity. When the space between the metallic terminals is wetted by urine, a potential difference is caused between them to generate an electromotive voltage, which operates an urinary incontinence detecting circuit to detect the urinary incontinence state, and a light emitting diode is operated to inform the patient itself or a helper of the urinary incontinence state. In this urinary incontinence detector, as a base current for turning ON the urinary incontinence detecting circuit is supplied by the electromotive voltage generated between the metallic terminals the consumption of the power source voltage by a battery can be suppressed, thus enhancing the life.
Japanese patent JP4138155 describes a local system. The care person must be in front of the patient to see the alarm. It does not have a radio system (RFID). This system is not based on a RFID technology which can transmit in a wireless way to a remote system, data (in particular the position) related to tagged objects.